One of the main challenges of building untethered microrobots is in the design of the propulsion system. Due to difficulties in storing energy at these scales, mechanisms are needed for harvesting energy from the environment for wireless applications.
While various methods of wireless sensing have been developed, few methods for wireless actuation have been documented. The lack of suitable miniaturized power sources is a major obstacle to their development. Wireless sensors are unique in that they need only to passively interact with their environment, a requirement that usually demands little or no energy. By definition, successful actuators must do work and expend energy. As forces and working distances increase, energy requirements also increase. Wireless actuators must receive their energy from their environment or from an outside source before they can expend it by acting on their environments. In addition to supplying power, methods for controlling the actuators must also be implemented to make the actuator useful. Depending on the application, actuators must be activated independent of neighboring actuators. A variety of drive methods, including piezoelectric, electrostatic, thermal and vibrational have been demonstrated in both wireless and wired systems. While many groups are working on microactuators few have reported work on actuators for robots.
Current wireless communication or power transmission methods typically involve inductive coupling between antennas. The efficiency of these methods decreases as the area of the coils involved shrinks, making inductive coupling a less attractive method for transmitting power to sub-millimeter microrobots. In addition to this power loss during transmission, such systems require onboard circuitry and electronics to drive the actuators. This leads to further inefficiencies and design challenges. One method of bypassing these problems is to transmit the energy directly to a mechanical structure, and use the structure to power the actuator or the structure is the actuator. This method has been documented in several papers where microrobots are operated in vibration fields or with actuators powered by externally applied vibrations. Other authors have reported on the remote control and propulsion of micro robots with magnetic fields. Most known methods rely on forces acting on permanently magnetized bodies in a gradient magnetic field. It is also known to move a magnetic screw through a medium by means of a rotating magnetic field.
Mei et al., proceedings of the 2002 IEEE International Conference on Robotics and Automation, Washington D.C., May 2002, p. 1131-1136, discloses a, much larger, swimming “microrobot” comprising a main body of 20 mm length with two resilient magnetic fins protruding from the body in different directions. Both fins bend in an external magnetic field to align therewith, and resume their original shape when the magnetic field is switched off. In an oscillating magnetic field, an oscillatory motion of the fins is generated, such that the device swims.
Ideally individual addressing would be a feature of the system design and not require integrated control electronics. Selectivity in wireless systems can be achieved if the system is designed to operate at resonance with different actuators being sufficiently separated in the frequency domain to be individually actuated by frequency-dependent power.
Rectification of oscillatory motion due to resonance is key to the operation of resonant actuators. Documented methods include impact, and ratchet-like behavior to convert oscillating motion to linear displacement or rotation in microsystems.
Although several sub-millimeter size microrobots have been proposed, none of these efforts have resulted in devices capable of performing useful tasks. Major deficiencies in these microrobots are propulsion and actuation. Few practical actuators exist that can generate sufficient force to perform tasks at these scales.